What’s the best way to keep a list of online users in a web application?

It depends on your definition of “online”—for this answer, let’s count anyone who has visited a page within the last 5 minutes.

I’d use redis—it performs 100,000 operations per second quite happily (since it’s all in-memory) so even with dozens of web avers just one redis instance should keep up perfectly happily.

One solution would be to use a redis sorted set. Every time a user views a page or performs an action, set a “score” in the sorted set to the current timestamp (the key should be their user ID). You can count the number of users who acted I the past five minutes using the ZCOUNT command with a timestamp of now-5mins. Periodically delete everything over than five minutes to free up memory.

You could also use redis sets one for every minute of the day, and then call SCARD on the five sets representing the last five minutes and add the results together.